Black medick

Black medick is a procumbent annual, biennial or short-lived perennial. It is native in grassy places and rough ground. It is prevalent in disturbed habitats and is a common weed in fields and gardens, including lawns. Black medick has a preference for fine textured, poorly-drained soils, low in organic matter with a pH from 6.5 to 7.8. It occurs in low vegetation that is cut or grazed.

In the past, black medick was chiefly found in temporary leys where it was introduced as a contaminant of the grass and clover seed. It was considered a useful plant on dry chalky soils and was often deliberately included in seed mixtures. However, as an annual it soon dies out leaving bare patches. It has been regarded as a valuable herbage plant and mixtures have been sown containing a high proportion of black medick but these were not popular in Britain. It has been cultivated as a green fodder for livestock, a green manure and as a hay crop. Black medick is grazed by sheep and has been introduced as a legume in sheep pasture.

The species is morphologically very diverse and many varieties have been described. It is occasionally found as a birdseed alien.

Biology:

Black medick flowers from April to August. The flowers may self- or cross-pollinate. Plants can flower within six weeks of emergence and may initiate new flowers throughout the growing season. Seeds on any one plant can therefore be found at different stages of maturity throughout the season. The pods are single-seeded. Seed number per plant may reach up to 6,600 but the average is 2,350.

Nearly mature seeds germinate readily and seedlings can emerge at any time of year. Seeds can remain non-dormant for around 10 days when almost ripe whether attached to the parent plant or not. Mature seeds possess innate dormancy and those with impermeable seed coats can remain dormant for many years. Scarification promotes germination but light is not required. Black medick seeds are said to germinate mainly in spring and autumn. Seed mixed with field soil and cultivated periodically, emerged mainly from March to May but odd seedlings emerged throughout the year.

In pasture, seedling emergence from freshly shed seeds is greatest when the vegetation cover is removed and seed predation is prevented. Seedling establishment also occurs when the vegetation cover remains intact but only if seed predation is prevented. Ants rather than birds or rodents are responsible for most of the predation.

Persistence and Spread:

Some black medick seeds are hard-coated and these can remain dormant for at least 10 years. Seed mixed with soil and left undisturbed declined by 87% in 6 years but in cultivated soil the decline was 93%. However, in another study, seed buried in mineral soil and left undisturbed retained only 1% viability after 4 years. Seed stored under granary conditions had only trace viability after 4 years. In dry-storage, seed gave 55% germination after 5 years.

Seeds can be dispersed by birds and by sheep and other grazing animals. Seeds can float in water for up to 5 days but viability is low after 9 months submergence in water.

Management:

Control is by good surface cultivations and hoeing out the plant to prevent seeding. Black medick has a thin wiry taproot and is difficult to uproot. In grassland, black medick plants can tolerate frequent mowing but not competition from tall grass.

Seed numbers in soil were reduced by 75% after a 1-year fallow and by over 90% when the fallow was continued for a second year. The land was ploughed, disked and harrowed during the fallow period. Seed numbers in soil under cropping with winter wheat over the same period increased in the first year then returned to the original level in year two. Fallowing at 5-year intervals over a 15-year period did not reduce seed numbers in soil because in the intervening cropped years this trailing weed was able to set and shed seeds before and after crop harvest.